In order to make certain kinds of measurements it is necessary that multiple geographically dispersed instruments all measure the signal of interest at the same time. An example of such a measurement is locating the source of a radio transmission by comparing the time of arrival of the signal at several different locations. By knowing the difference in signal arrival times at the various instruments (Time Difference of Arrival or TDOA) and the speed of radio propagation, one can calculate the source of the signal. For continuous signals (e.g. a commercial radio station) it is relatively easy to schedule all the instruments to take a measurement at the same time (and hence measure the same portion of the signal). For non-continuous and unpredictable signals (such as a hand-held push-to-talk radio) the problem is more difficult. Scheduling measurements at some future time does not necessarily result in useful measurements since it is unknown if the signal will be present at the scheduled time.
If the signal is strong enough that all the instruments (test devices) can detect when a transmission starts, a simple triggered measurement would suffice to capture synchronous measurements. However, in many situations one or more of the instruments may not “see” sufficient signal strength to reliably trigger. In these situations merely having each instrument trigger by itself is insufficient to make reliable measurements. Alternately, a first instrument can detect the beginning of a transmission and then communicate with the other instruments to schedule a synchronized measurement a short time in the future (how long in the future depends on the speed of the communication channel between the devices). This technique only works if the transmission length of the signal to be captured is longer than the delay between the instruments that are detecting the signal.